As an aluminum support for printing plate, particularly for offset printing plate, there is used an aluminum plate (including aluminum alloy plate).
In general, an aluminum plate to be used as a support for offset printing plate needs to have a proper adhesion to a photographic light-sensitive material and a proper water retention.
The surface of the aluminum plate should be uniformly and finely grained to meed the aforesaid requirements. This graining process large affects a printing performance and a durability of the printing plate upon the printing process following manufacture of the plate. Thus, it is important for the manufacture of the plate whether such graining is satisfactory or not.
In general, an alternating current electrolytic graining method is used as the method of graining an aluminum support for a printing plate. There are a variety of suitable alternating currents, for example, a sinewaveform, a squarewaveform, a special alternating waveform and the like. When the aluminum support is grained by alternating current supplied between the aluminum plate and an opposite electrode such as a graphite electrode, this graining is usually conducted only one time, as the result of which, the depth of pits formed by the graining is small over the whole surface thereof. Also, the durability of the grained printing plate during printing will deteriorate. Therefore, in order to obtain a uniformly and closely grained aluminum plate satisfying the requirement of a printing plate with deep pits as compared with their diameters, a variety of methods have been proposed as follows.
One method is a graining method to use a current of particular waveform for an electrolytic source (JP-A-53-67507) (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Another method is to control a ratio between an electricity quantity of a positive period and that of a negative period at the time of alternating electrolytic graining (JP-A-54-65607). Still another method is to control the waveform supplied from electrolytic source (JP-A-55-25381). Finally, another method is directed to a combination of current density (JP-A-56-29699).
Further, known is a graining method using a combination of an AC electrolytic etching method with a mechanical graining method (JP-A-55-142695).
As the method of producing an aluminum support, on the other hand, known is a method in which an aluminum ingot is melted and held, and then cast into a slab (having a thickness in a range from 400 to 600 mm, a width in a range from 1,000 to 2,000 mm, and a length in a range from 2,000 to 6,000 mm). Then, the cast slab thus obtained is subject to a surface-cutting step in which the slab surface is cut off by 3 to 10 mm with a surface cutting machine so as to remove an impurity structure portion on the surface. Next, the slab is subject to a soaking treatment step in which the slab is kept in a holding furnace at a temperature in a range from 480.degree. to 540.degree. C. for a time in a range from 6 to 12 hours, thereby to remove any stress inside the slab and make the structure of the slab uniform. Then, the thus treated slab is hot-rolled at a temperature in a range from 480.degree. to 540.degree. C. to a thickness in a range from 5 to 40 mm. Thereafter, the slab is cold-rolled at the room temperature to a predetermined thickness. Then, in order to make the structure uniform and improve the flatness of the plate, the thus treated slab is annealed thereby to make the rolled structure, etc. uniform, and the slab is then subject to correction by cold-rolling to a predetermined thickness. Such an aluminum plate obtained in the manner as described above has been used as a support for a planographic printing plate.
The present inventors previously proposed the enhancement of yield by continuously performing casting and hot-rolling from molten aluminum to form a hot-rolled coil of a thin plate, transforming the hot-rolled coil into an aluminum support through cold-rolling, heat-treatment and correction, and finally, graining the aluminum support (U.S. Pat. No. 5,078,805 which corresponds to JP-A-3-79798).
However, as a result of study-on the foregoing proposed methods, it was found that the average diameter, diameter distribution and shape of crystalline grains have a great effect on the adaptability to surface treatment and burning.